1. Field of the Invention
This invention relates generally to improvements in communication apparatus and, more particularly, but not by way of limitation, to communication apparatus for communicating between a first and a second object apparatus wherein the second object apparatus provides an emitted signal in response to a received transmit signal.
2. Brief Description of the Prior Art
In the past there have been many devices constructed to generate and transmit a signal, sometimes generally referred to as an interrogator and an interrogating signal, which is utilized to activate a second apparatus sometimes referred to generally as a responder, wherein the responder transmits a responder signal in response to the received interrogating signal. In some instances, such devices have been constructed to shape the responder signal in such a manner that the responder signal provides an indication of the responder identification. In many instances, the responder devices constructed in the past have been generally referred to as "passive", the term "passive" being generally utilized to designate a general type of transmitting apparatus which generates and transmits a signal in response to a particular, predetermined received signal.
There have also been interrogator-responder systems constructed in the past to interrogate responders located on various moving vehicles such as automobiles, trains and satellites, for example. One such system was disclosed in the U.S. Pat. No. 3,145,380, issued to Currie, wherein a passive responder was constructed to receive signals of different frequencies and transmit a coded signal in response to a particular interrogator signal. One other such device was disclosed in the U.S. Pat. No. 3,268,862, issued to Pettitt, wherein a roadside monitor transmitted distinct binary code data determined by the particular frequency of the transmitter of the roadside monitor, the vehicle responders being constructed to detect the discrete frequencies of the roadside monitor transmit signal to arrange the binary code data transmitted.
The U.S. Pat. No. 3,389,391, issued to Keller II, disclosed a vehicle identification system having a responder unit receiving an RF signal and transforming the received signal into a DC operating voltage for powering the responder transmitter which generated a series of pulses in a predetermined sequence, the binary code digits 0 and 1 being represented by two different tones and a third generated tone being utilized to indicate the beginning and the end of the binary-coded message. The U.S. Pat. No. 3,270,338, issued to Watters, disclosed an interrogator-responder system wherein the interrogator and the responder each included a pulse train generation means for developing a predetermined number of pulses spaced in time. These past systems thus generally utilize the interrogating signal to develop an operating voltage for the responder transmitter or, in some instances, a plurality of signal frequencies are utilized to activate predetermined portions of the responder or interrogator, the various past systems generally including automatic gain control circuitry, elaborate signal synchronization circuitry or a plurality of tuning circuits responsive to designated frequencies.